What is state variables ?

Explanation

Every equilibrium state of a thermodynamic system is described by specific value of some macroscopic variables called state variables.
An equilibrium state of a gas is specified by the values of pressure, volume, temperature and mass and composition if there is a mixture of gases.
A thermodynamic system is not always in equilibrium. A gas allowed to expand freely against vacuum is not an equilibrium state.

In a box partition shown in a figure (a), a gas is filled in its one side and there is no gas on the other side. If the partition is suddenly removed leading to free expansion of gas and pressure of the gas may not be uniform throughout.

A mixture of gases undergoing an explosive chemical reaction. In both situations, the gas is not in equilibrium and cannot be described by state variables.
In a cylinder as shown in figure, a mixture of gases undergoing an explosive chemical reaction is not an equilibrium state and here its temperature and pressure are not uniform.
Finally, the gas attains a uniform temperature and pressure and reaches thermal and mechanical equilibrium with its surroundings.